The Stat family of transcription factors is necessary for the growth and differentiation of mammary tissues. In mammary tissues, a significant means of Stat activation is the binding of the neuroendocrine hormone prolactin (PRL) to its receptor, the PRLr. PRL/PRLr binding results in the activation of receptor-associated signaling networks, such as the Jak/Stat pathway and the endocytosis and nuclear retrotransport of a complex between PRL and the prolyl isomerase cyclophilin B (CypB). The intranuclear PRL/CypB complex regulates Stat5-mediated gene expression and DNA binding. Using multiple approaches including yeast two-hybrid and transcription factor array analysis, we have demonstrated that the PRL/CypB complex regulates the interaction of Stat5 with multiple intranuclear proteins including the peptide inhibitor of activated Stat, PIAS3, the small ubiquitin-related modifier, SUMO2, and the transcription factor and proto-oncogene, c-Myb. It is the central hypothesis of this proposal that the temporal and spatial interaction of Stat5 with these proteins, as modulated by the PRL/CypB complex, regulates Stat5 function. This hypothesis will be tested by three specific aims using breast cancer and PRL-responsive cell lines both in vitro and in vivo. First, the structural basis for Stat5/PIAS3 interaction and the functional significance of this interaction will be mapped using complementary mutagenesis/overexpression and knockdown strategies. Second, the functional consequences of Stat5 sumolyation will be evaluated by cellular and biochemical approaches. Third, the association between Stat5 and co-activators/repressors, such as c-Myb will be assessed by targeted mutagenesis, small-interfering RNA (SiRNA), and chromatin immunoprecipitation (ChlP)-based methodologies. Given that our preliminary evidence indicates that the manipulation of these interactions results in the alteration of Stat5-mediated gene expression and the survival of breast cancer cells, our proposed studies are directly relevant to a better understanding of the pathophysiology of: human breast cancer, and as such, may provide the basis for novel therapeutic strategies aimed at modifying Stat function in this disease. [unreadable] [unreadable]